For pure JS (no trame), you should be able to do something like:
function print_state(wasm_id) {
console.log(wasmManager.getState(wasm_id));
}
wasmManager.cameraIds.forEach((camera_id) => {
wasmManager.sceneManager.addObserver(
camera_id,
"ModifiedEvent", // VTK Event to listen to
() => print_state(camera_id),
)
});
VTK has its own events which you can find here when clicking on vtkCommand from the AddObserver method.